In the United States, each resident discards approximately one waste tire annually. Discard of tires resulted in accumulation of approximately 242 million wasted tires nationwide in 1990 alone, exclusive of retreads. Waste tires are a minor portion of the solid waste stream yet represent a major disposal problem. While many waste tire incinerators have been designed, incineration may not maximize the potential economic recovery of energy and chemical materials from the tires.
Most tires are disposed of in landfills or in tire storage piles. However, whole waste tires are difficult to dispose of in landfills; they tend to collect gas, harbor rodents, snakes, and mosquitoes, pose a fire hazard, and drift upward in the landfill over time as other wastes consolidate and subside. Nonetheless, landfilling, stockpiling, or illegal disposal continue to be the primary methods of waste tire management.
Pyrolysis involves heating organic materials in the absence of oxygen to convert them to molecularly simpler and smaller organic compounds. When organic wastes such as waste tires are the feedstock, products of the process include char or carbon char, oil and gas. As early as 1830, a developer successfully commercialized an early application of pyrolysis involving the production of liquid products from wood. The production of coke from coal pyrolysis became the most common application of the technology; its use continues today. The use of wood pyrolysis to manufacture creosote oil expanded after the introduction of creosote as a wood preservative in 1838. Pyrolysis of coal and oil shales became common to produce oils in the United States and elsewhere in the mid-1800's. Pyrolysis plants to produce illuminating gas became common worldwide until the invention of the electric light bulb in 1879.
Pyrolysis processes may operate either as batch feed or continuous feed systems. Batch feed systems process a single charge of feed stock at one time. After the required residence time in the batch thermoreactor, solid products and residue are removed. Alternatively, in continuous feed systems, feed stock is conveyed through the thermal reactor, and solid products and residue are continuously discharged.
Pyrolysis relies on the addition of heat to break chemical bonds, providing a mechanism by which organic compounds decompose and vaporize. Most systems for pyrolysis of waste rubber and other hydrocarbons report operating temperatures in the range of about 480° F. to 1740° F. At temperatures above approximately 480° F., shredded tires release increasing amounts of liquid oil products and gases. Above 750° F., depending on the process employed, the yield of oil and solid tire derived char may decrease relative to gas production. Tires contain over 80% carbon and hydrogen, and these elements form the principle constituents of the solid, liquid and gaseous pyrolysis products. In addition to carbon-carbon bonds, decomposition of tires requires the breakage of sulfur-carbon and sulfur-sulfur bonds.
The solid product produced by most pyrolysis processes that use tires or other solid organic feed stocks is termed “tire derived char”, “tire derived carbon char”, or carbon black. This solid product can be further processed and cleaned to produce a higher grade of carbon black, or it can be marketed directly. Carbon blacks differ in particle size, surface area, average aggregate mass, particle and aggregate mass distributions, structure and chemical composition, and are rated according to industry standards, based on these properties. Conformity with industry standards determines the marketability of the tire derived carbon black. For example, intermediate quality virgin (un-recycled) carbon black is usually used in rubber products, while lower grades are used in products such as hoses and solid (not steel-belted) rubber tires. High purity carbon blacks are used in toner and for electronic sensors.
The surface area of carbon black has a substantial impact on quality and applications; carbon blacks containing submicron particles have a high surface area to volume ratio. The average particle size of a commercial carbon black ranges from approximately 5000 Å for a low cost thermocarbon to approximately 100 Å for the most expensive high color paint carbon. Also important is structure, as measured by aggregate size and shape, the number of particles per aggregate, and their average mass. These characteristics affect aggregate packing and the volume of voids in the bulk material. Void volume is one of the standards by which carbon black grades are judged.
Typically, tire-derived carbon chars produce carbon blacks having particles in the size range of 10 to 100 microns. This particle size range limits the ability of the material to be substituted for standard, less variable, carbon blacks containing submicron particles. However, there are some applications which use carbon black grades containing larger particle sizes, for example, use in plastics to improve weathering resistance, or to impart antistatic and electrically conductive properties. Char or carbon char material generated from pyrolysis can also be used as a source of fuel.
Tire pyrolysis also produces a gas that contains combustible concentrations of butane, propane, methane and ethane. Due to large amounts of carbon monoxide and carbon dioxide in the gas, the gas cannot be blended with natural gas for sale. Tire pyrolysis systems can also generate an oil-based liquid similar to Number 6 grade fuel oil; the liquid constitutes approximately 30% to 50% of the product derived from the organic content of the tire feed stock. Due to the high temperature and absence of hydrogen in the reaction mixture, most rubber pyrolysis methods produce a liquid that is high in polyaromatic hydrocarbons (PAH); these compounds are detrimental when released in the environment.
Many methods of pyrolysis have been described in the art using clay as a catalyst or non-sticking agent. See, for example, U.S. Pat. Nos. 1,680,908; 4,463,203; 4,300,009; 4,948,495; 5,114,541; 5,354,930; 5,233,109; and 5,114,541. These methods use high temperatures, in the range of 200°-800° C. (392°-1492° F.), and do not use negative pressure/vacuum conditions. Other patents have attempted to fine tune the pyrolysis process to favor production of certain products over others; see, for example, U.S. Pat. No. 5,359,061, which discloses pyrolysis of elastomers such as rubber tire scraps using molecular beam mass spectrometry techniques to detect decomposition products and to determine process parameters.
While some patents disclose the use of vacuum conditions, none disclose use of a vacuum in combination with the use of a catalyst. See, for example, U.S. Pat. No. 5,720,230 which provides a method of pyrolysis for discarded rubber tires, using temperatures between about 350° F. and 650° F., under slight vacuum conditions of one half to one inch mercury; and U.S. Pat. No. 4,740,270, which discloses treatment of used rubber tires by vacuum pyrolysis, at a temperature range of about 360° C. to 415° C. (680°-779° F.) under subatmospheric pressure conditions.
In most prior art methods the carbon black derived from pyrolysis of rubber tires has a high inorganic ash content. In addition, carbon black derived in this manner does not meet the industry standards as to particle size, purity and the like. None of the above described methods of pyrolysis of rubber tires and hydrocarbons generally are accomplished at lower temperatures and yet able to produce a higher quality of carbon black and liquid oil low in polyaromatic hydrocarbons as reaction products.